The present invention relates to an in-line electron gun for a cathode ray tube (CRT), and more particularly to a dynamic focusing electron gun for forming circular beam spots throughout a large, flat screen for a color CRT having a wide deflection angle.
Generally, an in-line electron gun for a color CRT is composed of a heater, a cathode and first through fourth grids. A picture is produced by the following sequence in the color cathode ray tube: (1) thermions (hereinafter termed electrons) are emitted; (2) the amount of emitted electrons is controlled by an external signal; (3) the electron beam is focused; and (4) the emitted electrons are accelerated to strike a phosphor screen.
In the above sequence, the first and second steps are performed by the cathode and the first and second grids (called a triode). When the cathode is heated by the heater, the electrons are emitted from the surface of the cathode. The emitted electrons pass through the passing holes of the first and second grids.
The third step is primarily performed by a main lens formed between the first and second focusing electrodes. The electrons having passed through the holes of the first and second grids are prefocused by a prefocusing lens formed by the second grid and first focusing electrode and are accelerated and focused by the main lens. Here, if the voltage applied to the first focusing electrode (focusing voltage) is controlled, the focusing state can be controlled so that a picture of the intended quality is realized on the screen.
The fourth step is carried out by a shadow mask and the second focusing electrode, that is, an anode, and graphite internally coated on the inner surface of the color CRT. Here, with the electrons being negative charges, a high positive voltage is applied to the anode so as to attract the electrons and cause them to strike a phosphor screen.
A conventional in-line electron gun for color CRT will be described with reference to FIGS. 1 and 2. In each of the attached drawings, the portion above the central dot-and-dash line shows the vertical half section of the electron gun, while the portion below the horizontal half section. The solid line indicates the path of the electron beam when directed at the screen center, and the dotted line indicates the deflection path of the electron beam around the periphery of the screen.
In FIG. 1, a conventional static focusing electron gun does not compensate for the over-focus in the vertical direction nor under-focus in the horizontal direction which are caused by a deflection yoke lens A2 and produced when the electron beam emitted from a cathode 2 is deflected toward the screen periphery by the deflection yoke. Hence, an electron beam spot formed around the screen periphery has a halo vertically and a thin laterally-elongated core horizontally. These deteriorate picture quality.
In order to improve the vertical halo around the screen periphery of the static focusing electron gun, a dynamic focusing electron gun with a quadrupole lens which is an auxiliary lens has been proposed. In the quadrupole lens, when the electron beam is emitted from the cathode 2 and is deflected toward the screen periphery, a dynamic focus voltage modulated by being synchronized with a deflection signal of the deflection yoke is applied so as to compensate for the oblique astigmatism and focal length of the deflected electron beam.
In FIG. 2 illustrating the conventional dynamic focusing electron gun, when the electron beach emitted from cathode 22 is deflected toward the screen periphery, the electron beam diverges vertically by a quadrupole lens B1 formed by second focusing electrode 26 and first and third focusing electrodes 25 and 27 to which a dynamic focus voltage Vd modulated by being synchronized with the deflection signal of the deflection yoke is applied. Then, after passing the periphery of a main lens B2, the electron beam proceeds in parallel. Passing the periphery of a deflection yoke convergent lens B3, the electron beam is under an intense convergent action, resulting in excessively and vertically converged electron beam spot. Meanwhile, since the electron beam is collimated by quadrupole lens B1, passes the central portion of main lens B2, is incident on deflection yoke divergent lens B3 at a small convergent angle and diverges by deflection yoke divergent lens B3, the electron beam spot becomes horizontally elongated, compared with the vertical beam spot.
Accordingly, in the conventional dynamic focusing electron gun, even though the vertical halo is not created, a moire phenomenon is evident on the screen, since the height of the electron beam spot is much smaller than that of the electron beam passing hole in the shadow mask. Moreover, the size of the horizontally-elongated core is nearly the same as that of the static focusing electron gun. As a result, the conventional dynamic focusing electron gun offers no real improvement.